1. Field of the Invention
This invention relates to a light-receiving circuit that enables to compensate a temperature dependence of an optical sensitivity of a photodiode.
2. Related Prior Art
An avalanche photodiode (APD) has a superior optical sensitivity and can respond to high frequencies, therefore, the APD may be used in a light-receiving module of an optical communication system. On the other hand, the APD shows a temperature dependence in its multiplication factor and, accordingly, affects quality of the optical and the electrical signal in the optical communication system. Particularly in the high-speed optical communication over 10 Gbps, such less quality causes transmission errors of the optical signal. The light-receiving circuit using the APD, therefore, a function of temperature compensation should be taken into account.
Various techniques to compensate the temperature dependence of the APD are reported. For example, Japanese patent published by 989-215140 has disclosed that the bias voltage supplied to the APD is adjusted by a transistor having a temperature dependence capable of compensating that of the APD. Another technique is shown in FIG. 1 that the bias voltage is corrected based on the temperature dependence of the thermistor.
Japanese patent published by 1999-205249 has disclosed a technique that the bias voltage is adjusted by the collector current of the transistor, the base of which is connected to the temperature sensor having an optional temperature co-efficient. The temperature sensor varies the base current so that the collector current thereof is changed.
However, a compensation circuit using the transistor or the thermistor is hard to adequately compensate the APD because such active devices show non-linear temperature dependence while the APD has liner temperature dependence. The optical bias voltage, at which the APD shows an optimal multiplication factor to a predetermined optical magnitude, is denoted by a linear function to the temperature T.
Moreover, the APD as one kind of semiconductor devices intrinsically has a scattered characteristic. For example, the APD applied in the optical communication system having 10 Gbps speed has nearly 10% scattered temperature dependence in its optical sensitivity. For such APD, adjusting the bias voltage so as to obtain an optical sensitivity affects the temperature dependence of the APD. The optimal sensitivity defined by the bias voltage applied thereto and the temperature dependence are preferably adjusted independently.
One object of the present invention is to provide a light-receiving circuit for the APD that solves aforementioned subjects, namely, the temperature dependence and the optimal bias voltage of the APD are independently compensated.